Generative or additive layer manufacturing methods are increasingly being used to produce prototypes or completed components very quickly. In contrast to conventional production processes, which comprise removing material from a block of material by, for example, milling, cutting, drilling or other machining processes, additive layer manufacturing processes construct a desired three-dimensional object directly layer by layer based on a digital description or representation of the object. They are also known as 3D printing or rapid prototyping.
In a typical additive layer manufacturing method a thin layer of material, from which the object is to be produced, is first applied to a carrier plate in powder form and the powder of the layer which has just been applied is melted or sintered using laser radiation selectively only in those areas of the layer which correspond to the object to be manufactured. A further thin layer of the material in powder form is then applied to the thus-processed first layer and in turn melted or sintered using laser radiation selectively only in those areas of the layer which correspond to the object to be produced. This step is repeated until the complete object has been manufactured. In each layer, the powder which does not correspond to the object is not irradiated and remains in powder form, with the result that it can be removed from the completed object at a later time. The carrier plate can be provided by a movable table which, after each irradiation of a layer, is moved downwardly by a distance which is identical to the thickness of this layer in order to ensure that the starting conditions are identical before each layer is applied.
It is to be pointed out in this connection that it is in principle also possible for the individual layers not to be continuous or to completely cover the carrier plate but to have material only in those areas which correspond to the object to be produced or in areas which comprise those areas which correspond to the object to be produced.
Specific additive layer manufacturing methods are the so-called selective laser melting (SLM) and the so-called selective laser sintering (SLS), in which, as indicated above, a laser beam is used to irradiate the layers. However, it is also possible to use a particle beam and in particular an electron beam for this purpose. Specific additive layer manufacturing methods which use an electron beam are, corresponding to the two processes mentioned previously, the so-called selective electron beam melting and the so-called selective electron beam sintering.
As explained above, the object is constructed directly layer by layer in a three-dimensional manner. This makes it possible to produce different highly complex objects efficiently and quickly in the same device from different materials, in particular from metal but also from plastics and ceramic materials. For example, highly complex grid or honeycomb structures, which cannot be generated, or can only be generated with difficulty, using other methods, can be easily produced. In comparison with traditional production processes, the complexity of the object has only a limited influence on the production costs.
In additive layer manufacturing methods such as those mentioned above it must be noted, however, that, in areas of the object which form an overhang or a projecting or cantilevered portion during the layer-by-layer construction (i.e. in the orientation of the object during its production) viewed opposite to the direction of the force of gravity, particular measures may have to be taken to enable the manufacturing of the object or to increase its geometrical precision. In such areas, which are designated as overhang in the framework of this application, a melted or sintered part of each layer extends, with an edge section or portion thereof, beyond the melted or sintered part of the previous layer such that these edge sections of the individual layers are not supported by a melted or sintered part of the respective previous layer. This leads to the areas sinking into the powder bed under their own weight if the individual edge sections in each case project too far beyond the respective previous layer.
A possible measure is to select the extension of the edge sections such that the step structure provided by the individual layers on the surfaces of the areas stabilizes these sufficiently to prevent the sinking in. The exact demands on this step structure depend, among other things, on the structure and the dimensions of the object and on what forces act on the overhanging areas as a result of this. However, it has been found that problems can be reliably avoided if the surfaces of the overhanging areas do not exceed an angle of 50° with respect to the direction of the force of gravity during the layered construction. It is to be noted that, in the framework of this application, as is customary the outline, extension or course of a surface disregards the step structure which is always present, i.e. represents an averaging over the step structure.
However, this condition cannot always be met and support structures must then be provided for the overhanging areas, which structures are mechanically or chemically removed once the object is completed, or remain in the object. In either case, support structures mean additional material expenditure and thereby increase the weight and the cost of the object.
An example in which the above condition cannot be met is a tubular element with, for example, a circular inner and outer cross section, the longitudinal axis or direction of extension of which extends at an angle of more than 45° or 50° and for example perpendicular to the direction of the force of gravity during the layered construction. Then the circularly closed inner surface of the tubular element namely necessarily has sections which border or define an overhanging area or overhang in the above sense and extend at least in parts at an angle of more than 50° with respect to the direction of the force of gravity. If the tubular element exceeds a certain internal diameter, the support within the tube wall itself is no longer sufficient, for the reasons given above, to be able to produce the element without the provision of additional temporary support elements inside the tubular element.